1. Field of the Invention
The present invention relates to an ink, an image recording process, an ink cartridge, a recording unit, an ink set and an image forming apparatus.
2. Related Background Art
Inks with greatly various manners of being composed have heretofore been reported for writing utensils (fountain pens, marking pens, aqueous ball point pens, etc.) and ink-jet recording. In particular, in recent years, detailed research has been conducted regarding such issues as composition and physical properties of carbon black itself, and compositions and physical properties of inks containing such carbon black, whereby good recording can be conducted even on plain paper such as paper for copying, paper for reporting, notepaper, letter paper, bond paper and continuous business forms which are commonly used in offices.
For example, Japanese Patent Application Laid-Open No. 3-210373 describes water-based ink-jet inks comprising acid carbon black and an alkali-soluble polymer as a dispersing agent for the carbon black. Japanese Patent Application Laid-Open No. 3-134073 describes ink-jet inks comprising neutral or basic carbon black and a water-soluble resin as a dispersing agent for the carbon black, which are easy to be provided as dispersions excellent in shelf stability and ejection stability in bubble-jet recording apparatus.
Japanese Patent Application Laid-Open No. 8-3498 has offered a technical problem that when an ink comprising carbon black together with a dispersing agent is used as an ink for ink-jet recording, ejection becomes unstable, or sufficient optical density is not achieved, and discloses inks using self-dispersing carbon black capable of being dispersed in a solvent without using any dispersing agent as inks capable of solving such a problem. WO-A-No. 96/18695 (Japanese Patent Application Laid-Open No. 10-510862 through PCT route) and U.S. Pat. No. 5,746,818 (Japanese Patent Application Laid-Open No. 10-95941) also disclose ink-jet inks comprising self-dispersing carbon black and describe them as creating high-quality images.
The present inventors disclosed, in EP-A-No. 943,666, a technical means that a salt is added in addition to the above-described self-dispersing carbon black upon use of the self-dispersing carbon black as an ink-jet ink for the purpose of improving dependency of image density on the kind of paper and reducing bleeding at boundaries between a black ink and color inks.
By the way, as factors for considering the nature of an ink for ink-jet, there are the intermittent ejection stability and crusting property of the ink. Specifically, the intermittent ejection stability of the ink means the following property. Namely, when an ink is ejected from a predetermined nozzle of an ink-jet recording head, the ejection of the ink from the nozzle is stopped for a considerably long period of time (for example, about 12 hours) and the ink is then ejected again from the nozzle, the resumption of ejection of the ink may not be stably conducted in some cases to disorder printing. As described above, the operation that the ink is ejected from the predetermined nozzle, the ejection of the ink from the nozzle is stopped for the predetermined period of time and the ink is then ejected again from the nozzle is referred to as xe2x80x9cintermittent ejection of inkxe2x80x9d, and unstable resumption of ejection of the ink is referred to as xe2x80x9cpoor intermittent ejectionxe2x80x9d.
The crusting property of the ink means the following property. Namely, when ejection of an ink from a nozzle is stopped for a long period of time (for example, several days or longer) and the ink is then ejected again from the nozzle, a recovery operation for removing an ink which has undergone an increase in viscosity or solidification within the nozzle may be required in some cases. A state that the resumption of ejection of the ink is not stably conducted due to the viscosity increase or solidification of the ink within the nozzle is referred to as xe2x80x9ccrusting of inkxe2x80x9d, and an ink of which a great number of recovery operations is required for achieving stable resumption of ejection is referred to as xe2x80x9cink poor in crusting propertyxe2x80x9d.
Ink-jet inks have been required to have extremely high properties with the formation of ultrahigh-quality images by ink-jet recording in recent years.
However, it may not be said that technical findings for providing images having high optical density (OD) and sharp edge and achieving ink-jet ejection properties such as intermittent ejection stability and crusting property as to pigment inks comprising the above-described self-dispersing carbon black as a coloring material, particularly black pigment inks are sufficiently accumulated, and so their behavior as inks for ink-jet recording is not completely clarified.
For example, the ink comprising the self-dispersing carbon black and a salt proposed by the present inventors is extremely useful for lessening the dependency of image density on the kind of paper and the bleeding at boundaries between a black ink and color ink(s), which are the original objects. However, the present inventors have carried out a further investigation with a view toward establishing far excellent ink-jet recording techniques. As a result, it has been found that the intermittent ejection stability and crusting property of the inks greatly vary according to the structure of a recording head used.
The structures of a recording head is expected to be diversified with the wide product development of ink-jet printers and spread of application fields of ink-jet recording techniques, which has given us recognition that technical development is required as to inks which exhibit good and stable ink-jet properties against the diversification of recording heads while making the best use of the excellent properties of the inks comprising self-dispersing carbon black.
It is accordingly an object of the present invention to provide an ink comprising a pigment, particularly self-dispersing carbon black as a coloring material and having excellent ink-jet recording properties.
Another object of the present invention is to provide an image recording process capable of stably forming high-quality ink-jet recorded images.
A further object of the present invention is to provide a method of improving crusting property in ink-jet recording using a head having, in an ink flow path, a site where the flow resistance of the ink is greatly changed.
A still further object of the present invention is to provide an image recording process and an image forming apparatus which can stably provide high-quality prints, and an ink cartridge and a recording unit used therein.
A yet still further object of the present invention is to provide a method of lessening crusting on an ink-jet recording head.
A yet still further object of the present invention is to provide a method of improving intermittent ejection stability in an ink-jet recording process comprising the step of ejecting an ink plural times at a prescribed time interval, and particularly a method of improving intermittent ejection stability at the time when a recording head of the so-called side shooter type that a liquid is ejected in a direction opposite to an ink ejecting pressure-generating element is used.
The above objects can be achieved by the present invention described below.
According to an aspect of the present invention, there is provided an ink comprising a coloring material and an aqueous medium wherein the coloring material contains a self-dispersing type carbon black, the self-dispersing carbon black having a hydrophilic group bound to the surface thereof directly or through another atomic group, and wherein the ink contains potassium ion in an amount of 0.6 wt % or more of the weight of the carbon black.
According to an embodiment of the present invention, there is thus provided an ink comprising a coloring material at least containing self-dispersing carbon black having at least one hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink has a potassium ion in a proportion of 0.6 wt % or more based on the weight of the potassium ion to the carbon black.
According to an embodiment of the present invention, there is also provided an image recording process comprising the steps of: applying energy to an ink; and ejecting the ink from a recording head towards a recording medium, wherein the ink comprises a coloring material at least containing self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, and wherein the ink contains a potassium ion in an amount of 0.6% or more based on the weight of the self-dispersing carbon black.
According to an embodiment of the present invention, there is further provided an ink cartridge, comprising a container containing an ink, the ink comprising a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, and the ink having a potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to an embodiment of the present invention, there is still further provided a recording unit, comprising an ink container containing an ink and a recording head portion for ejecting the ink in the form of ink droplets, wherein the ink is an ink for ink-jet comprising a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains a potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to an embodiment of the present invention, there is yet still further provided an image recording apparatus, comprising a recording unit having an ink container containing an ink and a recording head for ejecting the ink in the form of ink droplets by the action of energy, wherein the ink comprises a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains a potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to another embodiment of the present invention, there is yet still further provided an image recording apparatus, comprising an ink cartridge equipped with a container containing an ink, and a recording head for ejecting the ink in the form of ink droplets by applying energy to the ink, wherein the ink comprises a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains a potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to an embodiment of the present invention, there is yet still further provided a method of lessening crusting on a recording head, comprising, upon ejection of an ink from the recording head by applying energy to the ink, using an ink for ink-jet comprising a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to an embodiment of the present invention, there is yet still further provided an ink set, comprising in combination an ink comprising a coloring material selected from the group consisting of respective coloring materials for cyan, magenta, yellow, red, green and blue colors, and an ink comprising a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to another embodiment of the present invention, there is yet still further provided an ink comprising a self-dispersing pigment having a hydrophilic group as a coloring material, a counter ion to the hydrophilic group and another ion having a polarity opposite to that of the hydrophilic group and having lower hydrating ability than that of the hydrophilic group.
According to an embodiment of the present invention, there is yet still further provided a recording head with an ink comprising a self-dispersing pigment having a hydrophilic group as a coloring material, a counter ion to the hydrophilic group and another ion having a polarity opposite to that of the hydrophilic group and having a lower hydrating ability than that of the hydrophilic group.
According to an embodiment of the present invention, there is yet still further provided an ink tank containing an ink comprising a self-dispersing pigment having a hydrophilic group as a coloring material, a counter ion to the hydrophilic group and another ion having a polarity opposite to that of the hydrophilic group and having a lower hydrating ability than that of the hydrophilic group.
According to another embodiment of the present invention, there is yet still further provided an ink-jet ink comprising self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, a salt and an aqueous medium, wherein the ink retains the stable dispersed state of the self-dispersing carbon black even when the fluid state of the ink in a recording head has become a state of turbulent flow or a disturbed state.
According to another embodiment of the present invention, there is yet still further provided a method of lessening crusting on an ink-jet recording head equipped with an ink-ejection opening at a position opposite to an ink ejecting pressure-generating element in an ink-jet recording process comprising the step of ejecting an ink by means of the recording head, wherein the ink is an ink for ink-jet and comprises a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains a potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to a further embodiment of the present invention, there is yet still further provided a method of lessening crusting on an ink-jet recording head which has an ink flow path having a site where flow resistance of a liquid is greatly changed in an ink-jet recording process comprising the step of ejecting an ink by means of the recording head, the method comprising using, as said ink, an ink for ink-jet comprising a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains a potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to an embodiment of the present invention, there is yet still further provided a method of improving intermittent ejection stability in an ink-jet recording process comprising the step of repeating a process of applying energy to an ink to eject the ink from a recording head by an ink-jet system at a prescribed time interval, the method comprising using, as said ink, an ink for ink-jet comprising a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains a potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to another embodiment of the present invention, there is yet still further provided a method of improving intermittent ejection stability in an ink-jet recording process comprising the step of ejecting an ink plural times at a prescribed interval using a recording head from which the ink is ejected in a direction opposite to an ink ejecting pressure-generating element, the method comprising using, as said ink, an ink-jet ink comprising a coloring material containing a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains a potassium ion in an amount of at least 0.6 wt % or more based on the weight of the self-dispersing carbon black.
According to a further embodiment of the present invention, there is yet still further provided a method of improving intermittent ejection stability in an ink-jet recording process comprising the step of ejecting an ink plural times at a prescribed time interval using an ink-jet recording head having, in an ink flow path, a site where flow resistance of a liquid is greatly changed, the method comprising using, as said ink, an ink-jet ink comprising a coloring material a self-dispersing carbon black having a hydrophilic group bound directly or through another atomic group to the surface thereof, and an aqueous medium, wherein the ink contains a potassium ion in an amount of 0.6 wt % or more based on the weight of the carbon black.
The present inventors carried out various experiments on the above-described objects. As a result, it was found that there is a correlation as to ink-jet properties between the kind of a salt added to an ink and a structure of a recording head. In, for example, a recording head illustrated in FIG. 24, reference numeral 251 indicates an ejection-energy-generating element, 252 an ejection opening, 253 a feed opening, 254 a substrate, and 255 an ejection opening plate.
As apparent from FIG. 24, the direction of an ink-flow is greatly curved at a site of the feed opening, and the ink is moved in the direction of the ejection-energy-generating element. A value expressing the degree of curvature of the ink at the site is an angle xcex8 formed by the substrate at the site of the feed opening. When xcex8 is not higher than 90xc2x0, the ink is curved at an angle of substantially at least 90xc2x0. The present inventors found that in such a recording head, intermittent ejection stability, crusting property and the like of an ink greatly vary according to the kind of a salt contained in the ink.
On the other hand, no conspicuous difference in intermittent ejection stability and crusting property was observed between inks respectively containing different salts in a recording head of a structure scarcely having portions which disturb a flow of an ink. Salts yielding excellent results irrespective of the structure of the recording head were found to be all salts containing potassium, and inks containing a sodium salt or ammonium salt were poor in intermittent ejection stability and crusting property in the recording head shown in FIG. 24 compared with the potassium salt-containing inks.
The reason why the potassium salt-containing inks are excellent in intermittent ejection stability and crusting property even when using the recording head shown in FIG. 24 is not clearly known. However, it is considered from the above-described finding that flowability of an ink in a recording head is greatly affected by an salt contained in the ink.
Specifically, it is inferred that a difference in intermittent ejection stability and crusting property between the pigment inks is attributable to the fact that the behavior of cations added as salts in the inks, particularly in a recording head, greatly vary according to the recording head used. As the causes thereof, there can be considered the behavior of the cation added as the salt and the behavior of the ink itself.
The consideration by the present inventors as to the behavior of an ink using a potassium salt as the salt will be first described with reference to FIGS. 27A to 27C and FIGS. 28A and 28B. FIG. 27A schematically illustrate a state of self-dispersing carbon black in an ink containing the carbon black. In FIG. 27A, reference numeral 2701 indicates the self-dispersing carbon black, 2705 a counter ion to a hydrophilic group in the carbon black, and 2703 a water molecule. Self-dispersing carbon black 2701 retains its stable dispersed state by keeping the affinity for the water molecules present thereabout.
A case where a potassium salt is added to the ink, and a case where a sodium salt is added to the ink are illustrated in FIGS. 27B and 27C, respectively. A potassium ion exhibits negative hydration property as illustrated in FIG. 28B, while a sodium ion exhibits positive hydration property as illustrated in FIG. 28A.
When the sodium ion is present at a prescribed concentration in the ink, water molecules 2703 present about the self-dispersing carbon black are localized on the side of the sodium ions as illustrated in FIG. 27C. On the other hand, no great change in the relationship between the self-dispersing carbon black and the water molecules is recognized in the ink containing the potassium ion compared with the ink containing no potassium ion.
It goes without saying that these states are momentary states of the respective inks typically depicted. It is not that the inks are always situated in these states. It is inferred that the inks are often situated in these states from the viewpoint of probability. Accordingly, it is not that the dispersion stability of the carbon black in the ink containing the sodium salt is immediately lowered.
However, the recording head having the site i.e. curved portions, at which the main flowing direction of an ink is changed by 90xc2x0, as illustrated in FIG. 24 is considered to cause turbulent flow or disturbance in the fluid state of the ink. Such a fluid state has a possibility that the apparent viscosity of the ink may be raised. In such a recording head as turbulence is caused in the fluid state of the ink, it is considered that a difference in dispersion stability of the self-dispersing carbon black caused by such a difference in a state between the carbon black and the water molecules as illustrated in FIGS. 27B and 27C is actualized as a difference in intermittent ejection stability and crusting property.
The present inventors have been led to make the present invention based on these findings and considerations.
Regarding a pigmented ink, in particular, a pigmented ink containing a self-dispersing pigment, the present inventors have studied a sole and a combined influences of the above mentioned 2 points to the ink-jet recording, and the inventors have found that the dependency of the intermittent ejection stability and crusting property on the recording head""s structure can be reduced by adding potassium ion in an amount of 0.6 wt % or more based on the weight of the self-dispersing carbon black in the ink, and this is also effective even to a pigmented ink containing a self-dispersing carbon black to which a salt is added in order to alleviate bleeding between a black image and a color image. The pigmented ink according to the present invention can be handled much easier and quality of the image produced with the ink is satisfactory.